The operating performance of wireless telecommunication networks is of great interest to system operators. A significant area affecting the operating performance are handoff operations that occur when a mobile user moves from an originating cell to a neighboring cell. A handoff of the mobile user from an originating cell to a more suitable neighboring cell, in most systems is typically based on signal strength measurement of received signals. In analog telecommunication systems such as those operating in accordance with Advanced Mobile Phone Standard (AMPS), signal strength measurements together with supervisory audio tone (SAT) transmissions are used to determine the most suitable neighboring cell for handoff. In digital systems, such as those operating in accordance with Digital Advanced Mobile Phone System (D-AMPS) for example, the system routinely analyzes neighboring cells for an appropriate cell to handoff to. This analysis of neighboring cells typically relies on several criteria such as bit-error-rate (BER) measurements, propagation path loss measurements, and signal strength measurements i.e. signal integrity.
The integrity of signals are inherently affected by a number of factors such as obstructions such as terrain, building etc. which tends to cause log-normal fading. The combination of these factors tends to result in a received signal that is distorted by having fluctuating signal strength. FIG. 1 depicts a graph of signal strength versus distance from a base station (BS) for a typical transmitted signal received by a mobile station (MS). Curve 10 shows a theoretical representation of the undistorted signal in which the signal strength exponentially decreases with distance. Curve 11 is a representation of an actual signal containing fluctuations received the MS. These fluctuations can be significant i.e. as high as 3 dB or more even after filtering processes. Signal fluctuations have important implications for handoff operation i.e. inappropriate handoffs may be initiated when measuring the signal strength of a signal that is fluctuating. By way of example, a handoff is initiated when the received signal strength of a neighboring cell is greater than the received signal strength in the originating or serving cell. For a proper handoff event, this condition must remain true after the handoff in the neighboring cell. After handoff occurs to a neighboring cell, fluctuations in the received signal may result in a measurement in which the signal strength in the current cell is lower than that reported by the previous cell thereby causing an immediate handoff back to the original serving cell. The occurrence of an undesirable immediate handoff back to the original serving cell is referred to as an oscillating handoff. As an example, several oscillating handoffs may occur which are due to movement of the MS and corresponding signal fluctuations near the cell border.
One important parameter that can be set by the operator to reduce the rate of oscillating handoffs is the hysteresis level for the cell towards each neighbor cell. FIG. 2 illustrates the hysteresis associated with an exemplary omnidirectional cell. The output power of BS 12 will determine the location of the cell border 14 which therefore ultimately determines the size of the coverage area of the cell. Extending some distance further from cell border 14 is a secondary border 16 defined by the hysteresis. The depicted hysteresis is the difference between border 14 and border 16 which is essentially represented by segment 18.
an appropriate hysteresis level setting is an important factor in fine-tuning procedures to improve network performance. For example, a relatively low hysteresis level yields better handoff performance which is desirable for situations involving fast moving traffic e.g. along a highway. This is also desirable in microcell configurations where handoffs typically occur with high frequency. But unfortunately, a disadvantage with relatively low hysteresis levels is that it tends to increase occurrence of oscillating handoffs which thereby increases the processor load on the system, degrades speech quality and increases the risk of dropped calls.
On the other hand, a hysteresis level that is too large causes the cell to expand resulting in dragging handoffs i.e. lagging handoffs that can cause undesirable interference in neighboring cells. Furthermore, lagging handoffs may increase the risk of dropped calls due to weak signal strength. The cell expansion and interference effect is especially problematic in microcell configurations in high traffic areas where the already close proximity of the cells are particularly susceptible to interference. The increased interference may ultimately lead to lower voice quality and network capacity further degrading performance.
In the prior art, hysteresis levels are typically initially set to default value of around 3-5 dB which may be manually adjusted by studying traffic performance between cells. The technique for optimizing hysteresis can be tedious and labor intensive, especially in microcell environments where individual cells may require different hysteresis levels that are optimized in accordance with the traffic patterns experienced. Moreover, fixed hysteresis levels cannot dynamically maintain optimal levels when there are abrupt unanticipated changes in environmental conditions affecting signal transmissions. Another disadvantage of manual adjustment is that limited maintenance resources may result in unsatisfactory hysteresis settings in cells that require recurrent modification. In view of the foregoing, it is an objective of the present invention to provide a method of setting the appropriate hysteresis levels for reducing oscillating handoffs and improving network performance.